This invention relates to silver halide photographic paper for direct positives, and more particularly to silver halide photographic paper for direct positives which has high sensitivity and improved whiteness.
Recent years, fogged silver halide photographic papers for direct positives have a trend toward achievement of higher sensitivity. To achieve higher sensitivity, it is necessary to obtain sufficient development activity with impartment of fog as little as possible, and accordingly there has been advantageously used a method of imparting fog in the presence of a reduction fogging agent and a water-soluble gold compound.
As example of the method for imparting fog, there may be mentioned those described in U.S. Pat. No. 3,501,305, No. 3,501,306 and No. 3,501,307, etc.
On the other hand, as other methods for enhancing the sensitivity of fogged silver halide photographic paper for direct positives, there has widely been used a method in which an organic desensitizer adsorbed on the surface of silver halide grains is used as an electron acceptor. The above organic desensitizers include, for example, compounds described in U.S. Pat. No. 2,930,694, No. 3,431,111, No. 3,492,123, No. 3,501,310, No. 3,501,312, No. 3,567,456 and No.3,582,343, etc.
Meanwhile, it is a well known technique to add a brightening agent to photographic layers in order to increase the whiteness of finished photographic paper, and widely used are water-soluble brightening agents described, for example, in U.S. Pat. No. 2,933,390, Japanese Patent Publication No. 30495/1973, Japanese Unexamined Patent Publication No. 135833/1980, etc.
However, it was found by the present inventors that the sensitivity may extremely decrease when the water-soluble brightening agent is added in the direct positive silver halide photographic paper containing silver halide grains having been fogged in the presence of the reduction fogging agent and water-soluble gold compound. It is possible to minimize the decrease in sensitivity by adding in photographic layers the water-soluble brightening agent previously adsorbed in a water-soluble polymer having the mordanting effect, such as polyvinyl pyrrolidone, but the above method has been still insufficient for resolving the problem described above. Such extreme decrease in sensitivity caused by addition of the water-soluble brightening agent is a phenomenon that has never been observed in silver halide photographic paper of a usual negative type.